Overset Grids Technology in STAR-CCM+: Methodology and Applications

Transcription

1 Overset Grids Technology in STAR-CCM+: Methodology and Applications Eberhard Schreck, Milovan Perić and Deryl Snyder CD-adapco

2 Contents About STAR-CCM+ Overset grids method in STAR-CCM+ How overset grids work in STAR-CCM+ from user perspective Application examples: Parametric study: varying angle of attack Launching of lifeboats, missiles etc. Relative motion of vessels with crossing paths Store separation Other devices with moving parts Future development

3 About STAR-CCM+, I What distinguishes STAR-CCM+ most from other codes? User interface (everything under one roof ) Meshing tools: Surface wrapping Surface remeshing Surface repair Automatic volume meshing and mesh design Process automation Some physics and numerics features...

4 About STAR-CCM+, II Simulation of vehicle thermal management at Daimler AG: 777 solid parts, surfacewrapping and automatic mesh generation for a conjugate heat transfer analysis (3 years ago ). Mesh size: ca. 30 millions cells Meshing time: ca. 100 minutes High geometric fidelity Courtesy of Daimler AG

5 About STAR-CCM+, IV Many kinds of problems can be solved without overset grids Overset grids are needed for: - Easy parametric studies - Extreme or constrained motions Sliding grid (cylinder surface) Morphing grid

6 Overset Grids Method in STAR-CCM+, I Control volumes are labelled as: Active cells, or Passive cells. In active cells, regular discretized equations are solved. In passive cells, no equation is solved they are temporarily or permanently de-activated. Active cells along interface to passive cells refer to donor cells at another grid instead of the passive neighbours on the same grid... The first layer of passive cells next to active cells are called acceptor cells...

7 Overset Grids Method in STAR-CCM+, II Currently, triangular (2D) or tetrahedral (3D) interpolation elements are used, with either distance-weighted or linear interpolation... Other (higher-order) interpolations will come Background grid N 1, N 2, N 3 Neighbors from the same grid; N 4, N 5, N 6 Neighbors from the overlapping grid. Overset grid

8 Overset Grids Method in STAR-CCM+, III No explicit interpolation of solution is performed Solution is computed on all grids simultaneously grids are implicitly coupled through the linear equation system matrix...

9 Overset Grids Method in STAR-CCM+, IV Different interpolation functions can be used to express values at acceptor cells via values at donor cells (different interpolation elements) Interpolation elements are not unique when grids move, continuity is important Donor cells must be active cells. The change of cell status is controlled by the solver and happens automatically. The user can visualize the cell status as a scalar field (this can help in case of problems mostly due to inadequate grids)...

10 Overset Grids Method in STAR-CCM+, V Overset grids usually involve: One background mesh, adapted to environment (can be fixed or moving); One or more overset grids attached to bodies and overlapping the background mesh and/or each other. Each grid represents a separate region. Both background and overset mesh(es) can be generated in the usual way (or imported) as in the case of a single region. Each grid can also deform (e.g. in a coupled fluid-structure interaction simulation). Overset grid can fall out of solution domain (cut-out by boundary surface).

11 Example: Pitching Foil Overlapping grid can fall outside solution domain...

12 Overset Grids Method in STAR-CCM+, VI In the overlapping zone, cells should be of comparable size in both meshes (recommendation): Interpolation errors in the coupling equation should be of the same order as when computing convective and diffusive fluxes (interpolation over half a cell); The coarser of the two coupled meshes determines the error level. Between two body walls, at least 4 cells on both background and overset grid are needed to couple them (requirement). The overset grid should not move more than one cell per time step in the overlapping zone (recommendation).

13 Working with Overset Grids, I No compromises on usability: Any grid type can be used Most physics models can be applied Processing pipeline (meshing, solving, analysing) unaffected Minimum on additional set-up steps: New region interface (with interface options) New boundary condition Using STAR-CCM+ infrastructure for interfaces New intersector-module added (searches for donors, defines interpolation factors, cuts holes etc.) solver almost unaffected

14 Working with Overset Grids, II Background region Overset region Overset interface for regions Background and Over Set-up of overset grid computation of flow around a pitching foil in a channel: one background grid for the channel and one overset grid for the region around foil.

15 Working with Overset Grids, III Background region Overset region Overset boundary Front and back planes are symmetry planes. The overset region has one boundary that is fully submerged within background region... Overset grid surface has boundary type OversetMesh

16 Working with Overset Grids, V Volume Mesh Representation includes active cells used to plot results... Active cells in overset grid Active cells in background grid

17 Working with Overset Grids, VI Background Acceptor cells (value -2 ) Active cells (value 0 ) Passive cells (value 1 ) Checking Overlap Cell Status (scalar field): acceptor cells must separate active and passive cells direct contact is not allowed...

18 Working with Overset Grids, VII Over Acceptor cells (value -2 ) Active cells (value 0 ) Checking Overlap Cell Status (scalar field): the overset grid here contains only active and acceptor cells...

19 Pressure Contours Pressure contours with lines: small imperfections (two lines visible within overlap zone) visible only at few locations most contours are almost perfectly continuous (grid from previous slides)

20 Velocity Contours Velocity contours without lines: hardly any imperfection visible contours appear almost perfectly continuous (grid from previous slides)

21 Convergence of Iterations Residuals history for a laminar flow around an object Implicit coupling of grids allows convergence to roundoff level of residuals

22 Application to Parametric Studies, I Flow around a car at different angles of attack A horizontal section through both grids (only active cells are shown). Total number of cells: ca. 1 million Vertical section through the two grids (only active cells are shown).

23 Application to Parametric Studies, II Velocity distribution in a section parallel to bottom wall for different angles of attack

24 Application to Parametric Studies, III Residual history from the computation of flow around a vehicle in a wind tunnel at different angles of attack: time step 1000 s, rotation 15 per time step, standard k-ε turbulence model, under-relaxation 0.9/0.1/0.9 for velocities/pressure/turbulence, wind speed 40 m/s

25 Application to Parametric Studies, IV History of computed forces from the computation of flow around a vehicle in a wind tunnel at different angles of attack (since the time step is very large, steady-state solutions are obtained).

26 Application to Parametric Studies, V Simulation of motion of a container ship in Stokes waves propagating from right to left: initial vessel orientation 30 (upper) and -30 (lower) relative to the direction of wave propagation

27 Application to Parametric Studies, VI Simulation of motion of a container ship in Stokes waves propagating from right to left: initial vessel orientation 30 (upper) and -30 (lower) relative to the direction of wave propagation

28 Simulation of Lifeboat Launching, I Simulation by H.J. Morch, CFD Marine; Experiment by Norsafe AS

29 Simulation of Lifeboat Launching, III Simulation of lifeboat launching from a platform

30 Temperature Mach Number / Surface Temperature Simulation of Missile Launching

31 Vessels With Crossing Paths Two vessels with crossing paths: This kind of simulation would be difficult to perform without overset grids (sliding grids cannot be used; morphing would require frequent re-meshing)

32 Moving Control Surfaces

33 Store Separation, I Modeled as arbitrary point loads Defined through the GUI Custom functional relationships to match ejector force and stroke length Visualize loads real-time Note that initial motion is dominated by ejector forces

34 Store Separation, II

35 Store Separation, III

36 Store Separation, IV Comparison of motion in simulation and experiment

37 Store Separation, V Comparison of motion in simulation and experiment

38 Store Separation, VI t = 0.00 t = 0.16 t = 0.37 Comparison of pressure in simulation and experiment

39 Simulation of Flow in a Mixer, I

40 Simulation of Flow in a Mixer, II

41 Future Developments The most important future developments include: Allowing for multiple overset grids, overlapping each other (currently in progress); Automatic mesh adaptation to fulfil requirements of overset grids (avoid failures due to inadequate grids in the overlapping zone): Minimum number of cell layers in gaps Similar cell size in overlapping zone Refining the background grid ahead and coarsening behind a moving body, to facilitate simulation of motion over large distances.

42 Simulation of Pouring Thank you for your attention!